1. Field of the Invention
the present invention relates generally to the field of hydrocarbon removal and recovery from air-hydrocarbon mixtures, and more particularly but not by way of limitation, to processes for the effective and economical removal of hydrocarbon components from mixtures of air-gasoline vapors.
2. Discussion of the Prior Art
The earliest known installation in the United States of a vapor recovery system for gasoline vapors was a unit which incorporated a process combining compression and condensation. This early vapor recovery system was installed in the late 1920's on a gasoline storage tank, and while several commercial entities marketed vapor compression systems for storage tanks in the years following that initial installation, it wasn't until the late 1950's that vapor recovery systems were installed at truck loading terminals in this Country to meet local air pollution regulations which severely limited levels of hydrocarbon emissions. These systems also practiced compression-condensation processes.
Nationally, "clean air" legislation activity began to garner serious attention in the late 1960's, culminating in the Clean Air Act of 1968. This of course brought nationwide attention to gasoline vapor recovery and a marketing demand was immediately created to be filled by a host of manufacturers offering a variety of vapor recovery systems. With the exception of a lean oil/absorption system marketed by one such company, these systems were simply variations of the earlier compression-condensation units that were first installed for vapor recovery on storage tanks. The lean oil/absorption systems dominated the market until experience indicated that such units did not perform economically, even though theoretical design characteristics were favorable.
Cryogenic refrigeration systems began gaining market acceptance in the early 1970's as such vapor recovery units proved to be the more reliable of available systems. However, these units had high horsepower requirements, and required relatively high upkeep attention and expense. While meeting air pollution regulations of that era, the cryogenic units were, in general, not cost effective as such units represented an added cost of doing business.
An adsorption/absorption vapor recovery system was taught in U.S. Pat. No. 4,006,423, issued to McGill et. al. in January, 1978. This system represented a very large reduction in operating costs over the cryogenic units, for example, requiring only about ten percent (10%) of the connected horsepower of comparable cryogenic units. Further, the McGill et. al. vapor recovery system represented cost savings as the value of the gasoline recovered by these units provided a respectable return on investment. Furthermore, these units have enjoyed a very favorable history of lower maintenance costs. Consequently, various adsorption/absorption systems generally filled the market requirements during the late 1970's as truck loading terminals installed mandated emission control systems.
The trend of governmental regulations, implementing the Clean Air Act of 1968 and related laws, has been toward ever stricter standards and stricter interpretation of existing ones. Meanwhile, developments in the petroleum industry have increased the amount of gasoline vapors which must be processed by the vapor recovery equipment at loading stations. This latter phenomenon has been brought about chiefly by the elimination of lead constituents in motor fuels, the consequent increase in aromatic makeup of such fuels, and the increase of gasoline vapor pressures.
Due to difficulties encountered with uniform interpretation of regulations requiring the reduction of hydrocarbon emissions as a percent of the discharge vapor load, there has developed a trend toward specifying the amount of permissible hydrocarbon emissions in absolute values, that is, the rating of permissible emission levels in terms of weight/volume loaded, usually in milligrams of hydrocarbon per liter of liquid product loaded (and often calculated as gasoline equivalent for heavy liquids such as diesel product). As bottom truck loading became more prominent, with its accompanying leakage control difficulties, this approach to emission limitation was greatly resisted but has gained support as truck leakage has been greatly reduced. Currently, federal regulations require that total hydrocarbon emission be limited to less than 35 mg/liter in most parts of this country, and that trucks be periodically leak tested.
Of the vapor recovery systems presently in operation at truck loading sites in the United States, approximately sevety-five percent (75%) of same are either of the adsorption/absorption type taught in the above-mentioned McGill et. al. patent or of the cryogenic refrigeration design. Most of these vapor recovery systems were installed sufficiently far back in time that they no longer are adequately sized for their service locations. As noted, the amounts of vapor loading have increased, both due to the changed gasoline makeup and the reduction of vapor bypassing due to truck leakage, and the permitted emission limits have stretched the service loading to the point of exceeding capability in many cases. As a consequence, many truck terminals, faced with large capital expenditures for updating facilities, have simply been closed or consolidated with other such loading stations.
These circumstances have intensified the need for more cost effective vapor recovery systems. More particularly, there has arisen the need for a means to increase the capacity of existing adsorption/absorption recovery systems of the type taught in the afore-mentioned McGill et. al. Patent since these units, or variations of same, comprise the majority of vapor recovery units that are serving truck loading stations in this Country.
While the situation above described has dealt with gasoline vapor recovery in the United States, similar circumstances can be found throughout the free world as global attention has paralleled the development of clean air controls in this Country. An international mandate for hydrocarbon emission abatement has developed rapidly over the last few years.